Sound Designing with Winds, Part I

Nature whispers, Man shouts.

Sure, there will be the occasional storm but nearly all daytime ambiences will have winds quietly ebbing and flowing in the background. The secret of using Winds effectively is learning how to whisper, not shout.

By understanding the significance of wind in natural environments the sound designer can gain a high degree of control over the audience’s experience, even guide them back to the very origin of their existence. The trick is to pick the right wind then use it at just the right moment and at just the right volume.

Wind is by definition atmospheric turbulence. Since the atmosphere is the medium through which sound travels to the human ear, the presence of wind dramatically affects how far a sound will travel and how intelligible it will remain. For example, a snap of a twig may be crystal clear on a still night, but during a windy day it will be a dull click, if it is audible at all. Wind is, in effect, the valve that controls the information flow in bioacoustic systems. But before we learn how to open and close this valve, let’s explore the question, “What is information?”

Sound is information

Sound is created by events. These events may be a giant wave crashing on a pebble-strewn beach, a single raindrop rolling off a salal leaf, or the careful, predatory steps of an as yet unseen jaguar. At any given time there are often multiple events, each producing its own sound. Each sound carries information such as origin, forcefulness, familiarity. Each event-driven sound expands outward as a wave. This sound wave wraps around objects, reflects off walls, diffracts through air layers--goes through all kinds of changes--and as it does, it takes on new meanings such as distance traveled, spatial qualities, surface characteristics. In short, by the time a sound reaches the ear it is a new sound, an aggregated sound, full of vital information. In combination with the multitude of other sounds, this aggregated sound defines a place as unique as a thumbprint.

Daily cycle (opening and closing the valve)

As a broad generalization, all terrestrial environments share a common daily cycle of stillness and wind corresponding to the absence and presence of sunlight. Sunlight causes turbulence due to differential heating and subsequent mixing of atmospheric layers. Each day predictably starts out calm, and then becomes windy until after the sun sets, when calm conditions once again return.

Advancing weather fronts and seasonal transitions sometimes break this pattern, but as a broad and useful rule, stillness and windiness alternate over a 24-hour period.

Pre-Dawn (Valve full open)

Optimum conditions for sound propagation exist when the air is still and enough time has elapsed during the night to stratify the atmosphere by both temperature and humidity. The speed of sound differs depending on temperature and humidity, so under these conditions sound travels differently through each layer, sometimes confined within a single super-propagating layer, allowing sound to travel much further than one would expect. The pre-dawn condition is so fragile that distant sounds may become audible one moment and inaudible the next as these delicate air layers undulate in and out of stability. In fact, it is not uncommon for a nature sound recordist to setup during the previous night at what appears to be a pristine location only to later discover during pre-dawn that the noise from a distant highway is now clearly present.
While pre-dawn is the optimum time for message sending, few species vocalize during this splendid acoustic window, for the time is also ripe for eavesdropping by predators. The typical vocalizers during pre-dawn are species with nothing to fear.

Dawn (Valve just about to close)

But as the rising sun hovers on the horizon, diurnal species awaken and immediately take advantage of this safer window of opportunity. This music inspiring role call and is known as the dawn chorus, an orderly sequence of message-sending/receiving which may last anywhere from 15 minutes to as long as an hour. Species vocalize to establish territory, attract a mate, and to socialize. This same phenomenon reoccurs later in the day, albeit weaker, during the evening chorus. [We will learn more about this when Quiet Planet® releases Wild Voices later next year.]

Morning (Valve closing)

The still conditions which favored clear and long distance sound propagation deteriorate rapidly during the morning hours with the first whisper of wind caused by the warming rays of the sun.
The day’s first breeze is followed by a second, then a third, until one continuous, albeit sometimes highly variable, wind dominates. Leaves will rustle, branches may rattle, and rock outcroppings may whistle. Wildlife may continue to vocalize through the morning hours, but their song activity is weaker and less frequent than at dawn, though social contact calls may increase.

Mid-Day (Valve closed but leaking)

This is the dominant condition of daylight hours. Thermal layers are largely absent and wind cells exist, further reducing the ability of sound to propagate over great distance. In prairies, ridge tops, and other exposed areas, relatively strong winds frequently dominate. Species native to windy areas have adapted their songs to remain intelligible during otherwise unfavorable conditions by using frequency and amplitude modulations. Forested areas will roar at the treetops (sounding much like a river) even though conditions at the forest floor are calm. Winds continue to increase in amplitude during the day with the continued influx of solar energy.

Evening (Valve starts to open again)

Winds begin to subside once again as solar energy decreases during evening hours. Wildlife strike up the evening chorus, as once again conditions favoring message sending and receiving are increasingly restored; however, because the atmosphere has not yet reassembled into layers, the evening chorus is more subdued and less vibrant than the dawn chorus.

Night (Valve continues to open full)

Stillness returns gradually and eventually we hear the last breeze of the day. There may be an occasional zephyr, but for the most part, winds are now subsiding and the atmosphere is predominately still. (Except, of course, when a passing weather system whips up round-the-clock winds.)

Summary of cycle

Pre-Dawn has no wind. Dawn has occasional wisps. Morning has intermittent winds building gradually to the continuous winds of Mid-Day. Winds continue to build to their maximum strengths until the sun gets low on the horizon; then, as the sun falls below the horizon, we experience intermittent winds during evening. These winds eventually subside entirely at night, allowing the atmosphere to become stratified again and restoring optimum conditions for sound propagation.